1. Field of the Invention
The invention relates to a method and apparatus for the prevention of accumulation of water in comparatively low flow, low pressure wells.
2. Description of the Prior Art
The accumulation of liquids such as water in a natural gas well tends to reduce the quantity of natural gas which can be produced from a given well. As stated in paper SPE 2198 of the Society of Petroleum Engineers of AIME, authored by R. G. Turner, A. E. Dukler and M. G. Hubbard:
"In many instances, gas phase hydrocarbons produced from underground reservoirs will have liquid-phase material associated with them, the presence of which can effect the flowing characteristics of the well. Liquids can come from condensation of hydrocarbon gas (condensate) or from interstitial water in the reservoir matrix. In either case, the higher density liquid-phase, being essentially discontinuous, must be transported to the surface by the gas. In the event the gas phase does not provide sufficient transport energy to lift the liquids out of the well, the liquid will accumulate in the well bore. The accumulation of the liquid will impose an additional back-pressure on the formation and can significantly affect the production capacity of the well. In low-pressure wells the liquid may completely kill the well, . . . . "
There are many methods for the removal of liquids from a gas well. The simplest method of removing liquid from a gas well is to periodically blow the well down to a lower surface pressure, such as atmospheric pressure or the pressure in a storage tank. Perhaps the most common method of unloading a well with insufficient bottom hole pressure to allow production, is to run a small siphon string into the well and from time to time open the siphon string to atmospheric pressure. The small siphon strings might typically have a diameter of perhaps 1 or 11/4 inch. It also has been common practice to drill small "weep holes" siphon strings to enable gas to enter and prevent U-tube stallout during loading. The object behind the installation of this small tubing string and closing in the annulus is to reduce the production flow area and therefore increase the gas velocity in the smaller production string. The method can be used on low volume gas wells where a reduced production rate due to increased flowing friction is not a significant problem. Flow rates between 200 and 300 Mcf/d are required to lift liquid through a one inch tubing string at a surface flowing pressure of 600 psi and a surface flowing temperature of 540.degree. Rankine. This relatively simple solution results in the continuous production of both gas and liquid through the same producing string.
An alternative method employing small diameter siphon tubing strings is to produce gas up the tubing casing annulus and to periodically unload accumulated liquids by exposing the siphon tubing to low surface pressures. Accumulated liquids may also be removed through a siphon tubing by forcing liquids and gas up the siphon tubing through surface production equipment by periodically subjecting the casing tubing annulus to a high pressure.
These and other means of producing gas, despite the accumulation of liquids such as water in the formation, are described in a paper entitled "A Practical Approach to Removing Gas Well Liquids" by Hutless and Brandberry originally presented in 1971.
Differential pressure intermitters have also been used to unload gas wells. These devices measure the difference between siphon string pressure and casing pressure, determine the amount of water in the siphon tubing and blow the well when adequate load of water is detected. Gas is produced through the tubing casing annulus, and is slowly bled from the siphon string to cause water in the well bore to move into the siphon string. The pressure difference between the siphon string and the casing determines the amount of water in the siphon string. However, the efficiency of the differential pressure intermitter is dependent upon the bleed rate. If the bleed rate is too slow, liquids will build up in the casing. If the bleed rate is too fast, unnecessary amounts of gas are bled from the tubing and wasted to the atmosphere.
Downhole pumps can also be employed. In these installations liquid is pumped up the tubing and gas is produced up the tubing casing annulus. Downhole pumps caan be used to continue production in wells where the abandonment pressure is considered to be between 30 and 50 psi at the surface. Downhole pumping means are conventionally employed with wells which have been logged off and which can no longer be unloaded with siphon strings or intermitters. A typical downhole pumping unit comprises an electric motor, a pump, rods and other ancillary equipment.
Another means of unloading accumulated liquids is to employ a combination of a liquid diverter used with a gas lift valve. This system also produces gas through the tubing-casing annulus and lifts the liquid out through the tubing. The liquid diverter is employed to allow liquid to enter the tubing string which is open to atmospheric or separator pressure. After a sufficient quantity of liquid has entered the tubing string, the gas lift valve is actuated to lift this liquid to the surface, thus periodically unloading the well.
Finally, U.S. patent application Ser. No. 75,627 filed Sept. 14, 1979, now abandoned, discloses the use of a valve interspersed between the well output line and the production tubing to permit liquid and gas to be continuously transported through a secondary production conduit with only dry gas being produced through the primary production conduit. The formation pressure supplies adequate energy for this system.